This invention relates to the field of testing of packages for leaks, especially the testing of small packages used to house electronic components. The invention can also be used in other applications, such as in the testing of sealed packages which contain food products or medical devices, or which contain components to be used in outer space.
Micro-electronic components are mounted within packages that must be hermetically sealed to prevent contamination of the fragile inner circuitry and the very fine interconnecting wires or circuit traces. The cavity inside the micro-electronic package is filled with a dry, inert gas, and typically a metallic lid is brazed to the package to provide the hermetic seal. Leakage of molecules of water or oxygen into the package cavity can corrode or oxidize the interconnecting wires to the point that the circuit fails. In many cases, the loss of one component can lead to the failure of a subsystem or the failure of an entire system. The reliability of every electronic control system therefore depends on the reliability of each hermetic seal of each component package.
In the prior art, it has been known to use "tracer" gases to detect fine leaks in microchips. Typically, one places a batch of electronic components into a chamber, and pressurizes the chamber with a gas such as helium or krypton-85. Then, one removes the components, and lets them sit for a short time, allowing excess gas to dissipate. Then, the amount of gas still leaking from the package is measured with a mass spectrometer or radiation detector. In this method, it is often necessary to keep the components within the chamber for several hours, depending on the desired sensitivity of the test and the package volume.
To detect gross leaks, it has been known to place a batch of components in a tank filled with liquid fluorocarbons, while an operator watches for tiny bubbles streaming out of specific packages.
One disadvantage of both of the above methods is that only discrete components can be inspected. Components mounted to completed circuit boards cannot be inspected because of the quantity of the components and because the circuit boards themselves release gases.
The present invention provides an optical method of detecting leaks in small packages. The invention preferably employs electronic shearography as the method of interferometry, thereby making it possible to obtain test results in "real time". The same apparatus used to detect gross leaks is also used to detect fine leaks, although the methods are somewhat different. Also, both tests can be performed on components mounted directly on a circuit board.